Add support for passwords (i.e. support for badger encryption)

Its now possible to pass a password to badger-cli like this:

$ badger-cli -p some.db
Password:
some.db>  list
- foo
- foo2

We also support reading the password from stdin on *nix systems, e.g.:

$ cat mypasswordfile | badger-cli -p some.db
some.db>  list
- foo
- foo2